[TMC] 50 ohms vs. 70 ohms

Sheldon Daitch sdaitch at kuw.ibb.gov
Wed Sep 9 08:41:41 EDT 2009


An interesting experiment is to use an MFJ-269 or
similar antenna analyzer with some RG-59 cable into
a 50 ohm dummy load.  The VSWR will change
greatly depending on the frequency selected, and
returns to 1:1 at frequencies related to the wavelength
of the connecting cable.

Yes, there are Type N connectors for 70/72 ohms,
which have a different center pin diameter than the
center pin for 50 ohm Type N connectors.



Roy Morgan wrote:
> On Sep 8, 2009, at 3:21 PM, H.L. wrote:
>
>   
>> ....  How serious is the mismatch between 50-Ohm and
>> 70-Ohm systems and cables?
>>     
>
> The mismatch can be useful at times.  In particular, a piece of 70 ohm  
> coax between a 50 ohm one and a 90 or 100 ohm load can act as a  
> matching section/"transformer".  Collins used this principle in an  
> antenna.
>
> As I understand it, however, at frequencies below 30 mc, with  
> relatively low loss coax, mismatches, or running the coax at non 1:1  
> SWR levels usually does no harm.  (200 feet of RG-58 at 144 mc with  
> SWR at 3:1 gets you rather big losses, however.)
>
>
>   
>> Many TMC transmitters and linear
>> amplifiers are 70 ohms,
>>     
>
> 70  ohm coax shows the lowest losses, most all factors being normal  
> (SWR, dielectric losses, I squared R losses and so on).  TMC  
> transmitters are normally NOT amateur power levels, also.
>
>   
>> ...
>> useful one-foot up to ten-foot coax cables found at ham flea markets
>> are usually 50 ohms. Can we ignore the differences?
>>     
>
> Yes, especially at HF with short runs of cable.
>
> HOWEVER: the 70 ohm BNC connectors may damage the 50 ohm ones, or vice  
> versa. (The center pin diameter is not the same and can spread out the  
> female part, resulting in bad connections when the right connector is  
> used with it.)  I don't know of 70 o hm type N connectors, but if  
> there area any, the same warning would apply.
>
> Roy
>
> R
>   


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